I'd like to use both traces on my scope to look at 2 voltages that are without a common reference. E.g., across R1 & across R3:
________R1____ | R2 ... | R3 ____|______
I know that there are isolation probes to do this ($$$), but I'm wondering if there is some clever other way to do it. In my minimal experience I can't see one, but I'm hopeful that someone more clever than I has one to share.
Thanks, Bob
You could do it with a four channel cope that allows combining two channels in summing mode, like the cutom probes do. Can you just use two voltmeters, instead?
With that exact circuit you could do it because the voltage on R3 is some fixed multiple of the voltage on R2+R3 but in the general case, no, an ordinary 2 channel scope can only measure voltage between three points (one of them being ground)
You could buy or build a differential probe.
If you only wanted to see one trace (e.g. the voltage across R1), you could put the scope into summing mode and invert one channel. This doesn't work all that well because the gain of the two channels and probes isn't all that well matched.
Could I use a pulse transformer for isolation? E.g.:
I need to see the waveforms.
Differential probe: too much (this is just a lower-level hobby). Whole point is to see both signals.
Is this a repetitive signal, perhaps you could use the trigger input and probe the other two signals separately?
I think a big reason differential probes cost so much is because they are usually designed for mains voltage applications and are therefore built to meet various electrical safety standards (CAT ratings, etc.).
If you don't care about this safety aspect (e.g., you're probing some low voltage electronics) there appears to be some mass-produced ~US$50 option readily available on aliexpress (no clue about its quality).
If you are not too fussed about probe performance then maybe you can just hack together something on the bench. Perhaps a very simple opamp differential amplifier circuit like this one will fill the bill:
I did suggest you might build one. Depending on what your requirements are, it might be cheap and easy to do. I made a simple one, I think from an AD830.
"Not too fussed about performance" - that's me! Low frequency, low voltage, low precision is my world.
The differential amplifier looks good - thanks.
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